• Cabrera, Michel
• Semet, Vincent
• Gerges, Tony
• Charmeau, Jean-Yves
• Lombard, Philippe
• Cauchy-Clerc, Hugo
• Allard, Bruno

Abstract

Conventional PCBs are planar, but new 3D Plastronics processes, especially In-Mold (Structural) Electronics (IME/IMSE), aim to create non-planar devices. In IME, conductive inks (e.g. silver-based) are printed on a thin polymer film (e.g. 375µm PolyCarbonate). Surface Mount Devices (SMD) are connected with conductive glue. The circuit is thermoformed into 3D shapes and overmolded with thermoplastic via injection molding, enhancing strength, electrical insulation, and humidity protection. This enables the production of Human-Machine Interfaces (HMIs) featuring sensors, haptic feedback, and curved screens, drawing industry interest. While PolyCarbonate (PC) remains a benchmark in IME development, there is growing interest in bio-based, biodegradable polymers like PolyLactic Acid (PLA). Unlike PC, which relies on petrochemicals, PLA is primarily derived from corn starch and degrades in composting environments. It is also chemically recyclable, paving the way to a circular economy. However, PLA presents challenges due to its different mechanical and thermal properties compared to PC. The primary concern is PLA's lower glass transition temperature (55-60°C) versus PC's (around 140°C). However, our research team previously reported promising IME results with PLA [1]. IME inks, composed of silver particles, a polymeric binder, and solvent, necessitate curing to evaporate the solvent. Typically, this is achieved through heating in an oven, e.g., 20 minutes at 120°C for Dupont ME603 ink on PC film. PC's high heat resistance (HDT ~135°C) allows this, but not PLA. In our previous research [1], PLA-based conductive ink required a 2.5-hour heating at 55°C, yielding suboptimal conductivity. The same challenge applies to curing conductive glues used for SMD connections, requiring sufficient time and temperature. Here, we present an improved, faster curing method for conductive ink and glue on PLA. A detailed comparison with IME on PC is performed. This method, known as photonic curing, employs intense, brief flashes of high-intensity light to heat the targeted area. The distinct absorption spectra of the ink (or glue) and the polymer substrate ensure precise curing without compromising substrate integrity. We will present our results on using photonic curing compared to the traditional curing in an oven. We will show how the process improves the conductivity of the ink and how it impacts the thermoformability of the part. Quantitative adhesion tests on photonic curing of conductive glue are reported (to avoid soldering on PLA parts). Potential applications for connecting the SMD with a promising industrial process are discussed.

Topics

• impedance spectroscopy
• surface
• silver
• glass
• glass
• strength
• glass transition temperature
• injection molding
• thermoplastic
• drawing
• curing
• heat resistance